Most of the currently used toners for developing an electrostatic latent image comprise particles of a binder resin having a colorant and a charge control agent and other additives dispersed or otherwise incorporated therein. Magnetic toners, which use fine magnetic particles instead of the colorant or which comprise fine magnetic particles together with the colorant, are also used on a commercial scale.
The properties of the binder resin have a predominant effect on the properties of the resulting toner. Various synthetic and/or natural resins are used as the binder resin, but the modern high-performance copiers have rigorous requirements to meet with respect to development and other electrophotographic processes, and the binder resin to be used as a toner has its own rigorous requirements to satisfy. Since it is difficult for a single resin to meet the requirements, a mixed resin comprising a plurality of resins having different characteristics has been proposed. It is generally agreed that using such mixed resin in a binder resin is effective in improving the properties of the toner such as image fixing ability, charging ability and charge retaining ability, increasing the grindability of the resulting resin, as well as in preventing the toner filming that takes place when some toner particles are stuck onto the surface of a charge retaining member and carrier particles during development.
When the mixed resin is used in a binder resin, the individual resin components must form an intimate mixture. Otherwise, the visible image obtained by developing has fog or is fixed inadequately and, in addition, the resulting toner particles do not have great durability.
The process for producing toner particles generally consists of four steps: (1) a preliminary mixing step wherein a resin in a granular or other particulate form is mixed with a colorant and other additives; (2) a melting and kneading step wherein the mixture is put into a kneader where it is melted and keneaded to have the colorant and other additives dispersed in the resin uniformly; (3) grinding step wherein the resulting compound is ground into fine particles; and (4) a classification step for obtaining toner particles within a predetermined range of grain size. In the conventional technique, two resins that make up a binder resin are blended in the preliminary mixing step. Although the individual resin components are mixed further in the subsequent melting/kneading step, a completely uniform mixture is not obtained by the conventional technique since only mechanical and thermal means are used to mix the individual resins. If the resulting resin mixture is used as toner, a visible image with fog or one which cannot be fixed adequately is formed as a result of development, and the advantages of using two resins are lost. To provide an intimate resin mixture extended kneading is necessary and yet the resulting mixture is not completely uniform. The kneading operation itself becomes difficult if the resins have greatly different melt viscosities or if they have little miscibility with each other.